1. Field of the Invention
The present invention concerns a sample-and-hold unit, the sampling switch of which is a field-effect transistor. It is characterized in that the transistor is optically controlled, either by means of a semiconductor laser directly coupled to the gate region of the transistor or by means of a laser and an optic fiber. The optical control of the transistor, which is made of group III-V materials such as GaAs or InP, enables the sample-and-hold unit to work at frequencies of the order of at least 1 GHz.
2. Description of the Prior Art
Transistor-using sample-and-hold units, such as the one shown in FIG. 1, are well known. An analog signal (symbolized by a sinusoid) is applied to the input 1 of the sampler, at the source of a transistor 2 which acts as a switch. This transistor often is a field-effect transistor, and it may be made of silicon or of III-V materials such as GaAs, depending on the working frequency. When the transistor 2 is conductive, the signal charges a storage capacitor 3. At the output 5, an output interface 4 delivers sampled values which, depending on the nature of the interface, are amplified or digitalized or mixed with other values etc.
The transistor 2 is controlled at its gate by a train of pulses represented by a voltage V.sub.i. The gate and the source are joined by a load R so that, in a tracking period, the gate and source potentials are equal. In this standard type of system, the electron/hole pairs are created electrically.
In fact, this system which is perfectly suited to relatively low frequencies (in megahertz),has defects that get aggravated with the rise in frequency. For, the voltage V.sub.GS between the gate and the source, which controls the opening and closing of the channel, is the sum of the voltage of the analog signal and of the pulse voltage V.sub.i. Depending on whether the analog signal is sampled at a point where its voltage V.sub.M is close to the maximum or at a point where its voltage V.sub.m is close to the minimum, the voltage V.sub.GS varies because V.sub.M +V.sub.i =V.sub.m +V.sub.i. The result thereof is uncertainty over the switching instant of the transistor 2, hence uncertainty over the instant of sampling of the analog signal, the result of which is that the transistor loses its linearity with the rise in frequency.
Besides, the development of optic fibers to transmit information has led to a growing interest in having systems that can be directly controlled by light. Furthermore, the optical control of devices has a great many advantages such as the absence of electrical coupling and the insulation of the control circuit, the low amplitude of the optical control signal (some microwatts) compatible with the output levels of semiconductor lasers or electroluminescent diodes and immunity to jamming.
Now, it is known that materials of the III-V group such as GaAs, InP and their ternary and quaternary derivatives are sensitive to a light radiation of an appropriate wavelength: this opens up prospects of using systems in which integrated circuits simultaneously process optical signals and electronic signals.